Install git

You can likely install git easily via your native package management system, using a graphical interface or the command line. Try something like:

apt-get install git-core

or:

yum install git-core

If that does not work, try using a graphical tool or consult your distribution documentation.

Get NXT Improved Firmware sources

Now that git is installed, it is easy to get the sources.

By accessing NXT Improved Firmware, you accept the license.

Now open a terminal and type:

git clone http://git.ni.fr.eu.org/nxt-firmware.git/

That's it! You can now browse source code.